Automatic digital error detector for radar range tracking

ABSTRACT

This invention pertains to a new method of obtaining range, AGC and angle tracking error voltages employing digital methods of processing the receiver output of high range resolution monopulse tracking radars to generate these signals.

Ullitfid States Patent 1 Howard et al.

[ 1 Feb. 27, 1973 [54] AUTOMATIC DIGITAL ERROR [56] References CitedDETECTOR FOR RADAR RANGE UNITED STATES PATENTS TRACKING I [75]Inventors: Dean D. Howard; David C. Cross, a jtgif f both of 3:550:l2612 1970 Van Hijfte et al..... [73] Assignee: The United States ofAmerica as 3,646,588 2/ 1972 Van Popta ..343/7 3 represented by theSecretary of the y Primary ExaminerMalcolm F. Hubler 22 i g -i123 1971gitijomg-R. S. Sciascia, Arthur L. Branning and P.

0 nm er [211 App]. No.: 136,946

[57] ABSTRACT [52] 343/5 zgf g This invention pertains to a new methodof obtaining range, AGC and angle tracking error voltages employ- [511hm Cl G015 9/22 ing digital methods of processing the receiver output ofg range resolution monopulse tracking radars to [58] Field of Search..343/5 DP, 7.3, 7.4, 16 M generate these Signals- 4 Claims, 5 DrawingFigures DIGITAL PRotzssoR m m I f 340 as A0 42A I AZlllUTH ANGLE I SPEEDE Azmum I MIXER IF MLIFIER sAuPLE AND 0 DIGITAL ERROR l4 |aa 223 Now I:I PROCESSOR I MIXER -I IFANPUFIER I- f I g g 425 l 4* Mum ANGLE HIGHSPEED I ELEVATION I sANPLE AND P 5 menu. ERRoR I Q51? IF AMPLIFIER ERRoRDETECTOR I How L AIID PROQSSOR I II we 22c 26 34c l 5 E 44 l :3; :11RANGE vmzo I $3 5 2 I; L; 0161;; OIIIANGE I I DETECTOR I HOLD YPROCESSOR I I 20 24 I I I I LOCAL AGC VOLTAGE L m I I I OSCILLATORMPLlFIER I @IERATORl I I D-A 46 Two AXIS I L AzmuTN ANo j WTEM I IELEVATION sERvo TRANSMITTER TIMING ANTENNA I cmcuns I 485 I 0R|vE sYsTEuL 2 o-A I fi I w D-A I L fl 1 PATENTEUFEBZTISTS SHEET '4 OF 4 z 5E, 5%Us 5% 28% m 35 52 ha 2: 5%

.To: mm m n wt INVENTORS DEAN D. HOWARD DAVID C. CROSS a (If Haw PS] II5.4a wwzg AUTOMATIC DIGITAL ERROR DETECTOR FOR RADAR RANGE TRACKINGSTATEMENT OF GOVERNMENT INTEREST The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

BACKGROUND OF THE INVENTION Radars used in target tracking arecontinuously measuring the delay between the RF pulse transmission andthe receipt of the echo returned from the target and relative anglebetween the target and the antenna axis. A range tracker is usually usedto obtain this delay information and to position the range and angleerror detectors about the target. Many of the range trackers employearly-late or split gate range error sensing circuits which attempt todivide the target area into two equal areas which bracket the target inequal amounts. When either the early or late portion of the target echoreceived is greater in one gate than in the other gate, an error outputis produced which is used to reposition the early and late gates in timein order that the next range and angle error measurement will be madeabout the target. This system of early and late gates is simple andefficient for operation with conventional pulse lengths of 0.1microseconds or greater. However, with the ad vancing state-of-the artmaking high range resolution monopulse practical with very short pulselengths, short compared to target length, on the order of a nanosecond,the analog approach of the early-late gate became more complex. Highrange resolution tracking radar will typically employ rapid samplingtechniques thereby lending itself to digital processing techniques.

SUMMARY OF THE INVENTION The general purpose of this invention is toprovide a range tracker for a monopulse tracking radar which usesdigital sampling and processing techniques to produce the range andangle error signals which are used in tracking control and an automaticgain control signal (AGC). The invention includes a high speed sampleand hold circuit which divides the wide band video into a number ofsampled segments each of which stores a portion of the wideband targetvideo. This stored information is analyzed by a digital processor todetermine the range, AGC and angle error voltages; and provide a digitalor analog output representative of the targets change in position sincethe last stored range, the amount of angle error, and an AGC signal. Therange error output is used to correct the last stored range positionwhile the angle output is used to direct the antenna toward the target.

OBJECTS OF THE INVENTION It is therefore an object of the invention toprovide an improved range tracker for monopulse radar.

Anotherobject is to provide an improved angle error detector formonopulse radar.

Yet another object is to employ digital processing techniques to processthe range and angle wideband video and produce an output which is anindication of the range error, an average amplitude of the receivedsignal and the angle error for use in a monopulse radar.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows the preferred embodiment of theinvention;

FIG. 2 shows the error detection circuitry of the preferred embodiment;

FIG. 3 shows a flow chart of the processing techniques for range errorand AGC;

FIG. 4 shows a flow chart of the processing technique for either theazimuth or elevation error.

FIG. 5 shows a comparison of the digital techniques with conventionalearly-late gate range tracking.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before proceeding with thedescription of the present invention, it is considered advisable todiscuss some basic concepts of tracking radars. Tracking radars are usedprincipally for continuously measuring the position of a target inrange, azimuth angle and elevation angle. These radars typically consistof an antenna, which radiates a pencil-beam, from a rotating platformwhich is controlled in azimuth and elevation position by angular errorsignals which cause the antenna to change position keeping the targetcentered in the beam. These radars would typically employ a displaywhich shows a target as a single point source of brightness on a CRTusually indicating the range; range and azimuth; or range, azimuth andelevation of the target depending on the type of display.

The accurate tracking of a target, when less highly resolved range andhigh resolved angular position are employed, is complicated by anglenoise (also called glint or scintillation), which causes the angularlocation of an apparent source to wander back and forth about the longtime average radar center of the target, due to echo signal from acomplex target being dependent 'upon the relative phase and amplitude ofthe component echo signals and the relative angular locations. This longtime average radar center is herein called the true center of thetarget.

The present invention employs highly resolved range and wide bandwidthangular error information which is supplied to the range trackingsection of the radar wherein the range error is detected, the timingcircuits of the radar are adjusted for the new range, and the angleerror outputs are supplied to a two axis servo system which drives theantenna. Now referring to FIG. 1 which is conventional monopulsetracking radar wherein the digital processor in essence replaces thebox-car circuits. These box-car circuits are normally used in theazimuth, elevation, AGC generator, and range error detector. Againreferring to FIG. 1, the overall monopulse tracking radar is shown as10. A transmitter 12 supplies a short pulse, on the order of a fewnanoseconds, to the feed horn 14 of a cassegrain antenna 16. The RFsignal reflected from the target is received by feed horn 14 whichproduces a summation output coupled to mixer 18C, an elevationdifference output coupled to mixer 18B and an azimuth difference outputcoupled to mixer 18A. Local oscillator 20 is connected to mixers 18A, Band C. Mixers 18A, B and C are connected to IF amplifiers 22A, B and C,respectively. The gain of the IF amplifiers 22A, B and C is controlledby AGC voltage amplifier 24 which receives information from the rangevideo detector 26, which may be an amplitude detector. The output fromIF amplifier 22C is also connected to azimuth and elevation angle errordetectors 28 and 30 which may be phase sensitive detectors. IF amplifier22B supplies elevation difference information to error detector 30', andamplifier 22A is coupled to detector 28. The output of the range videodetector 26 is a unipolar wide range video, (see FIG. 3). While theoutput from the angle detectors 28 and 30 are bipolar video, (see FIG.4), giving indications of the angular error from the boresight axis ofthe antenna. The outputs of error detectors 26, 28 and 30 are coupled tothe digital processor 32. Each of the detectors is connected to aseparate high speed sample and hold circuit 34A, B and C. These circuits34A, B and C are controlled by the system timing circuits 36 whichproduce a pulse to start the sample and hold circuits just before atarget echo is expected. Sample and hold circuits 34A, B and C areconnected to multiplexer 38, the output of which is coupled to A and Dconverter and memory storage unit 40. The stored memory information issupplied to azimuth and elevation digital error processors 42A and Bwhich are similar in nature. Range information is applied to the digitalrange error processor 44 and digital AGC generator 46. The output of theazimuth digital error processor 42A is supplied to D to A converter 4%Dwhich controls the azimuth servo drive portion of the antenna drivesystem 50. The elevation digital error processor 428 supplies an outputto the D to A converter 48C which controls the elevation portion of theantenna drive system 50. The digital range error processor 44 is anoutput representative of the range error coupled to the D to A converter488. The analog output of the converter 488 controls the system timingcircuits 36 which control the time at which a pulse is generated to thesample and hold circuits 34A, B and C. When a signal is received by thesystem timing circuits 36 indicative of an increase in range, the timingcircuits delay the start pulse to the sample and hold circuits 34 by theincreased amount indicated by range error processor 44. The systemtiming circuits also supply a trigger to the transmitter to generate theRF pulse. This is independent of the timing signal to the sample andhold circuits while the sample and hold circuits are delayed withrespect to this trigger pulse. The output of the digital AGC generator46 is coupled through D to A converter 48A to the AGC voltage amplifier24 to control the gain of the IF amplifiers 22. Thus, FIG. 1 shows theoverall preferred embodiment of the invention.

Now referring to the operation of the digital processor in determiningthe range error and AGC voltages necessary for tracking, FIG. 3 shows aflow chart of the processing techniques for the V and V voltages. Theinvention requires that the envelope of the target return or echo, shownin FIG. 3 as wideband video return 60, be sampled and stored in adigital format. As the video need only be sampled in the vicinity of thetarget, the range tracking system of the radar will provide a gate tostart the sampling in the proper area. The samplers 34A, B and C may beof a type similar to those used in sampling Oscilloscopes. An evennumber of samples, n, should be taken at intervals at least as small asthe resolution cell of the radar and the size of the target. Forexample, if the resolution cell is 1 foot and the size of the target isa foot aircraft, n should be greater than 120. The size of n would ofcourse be controlled by programming the digital processor and determinedby the size of the target. After the range video has been sampled, thereis an array of stored data, R, of size n available for processing. Thestored data 1 through n is shown in FIG. 3 as 62 thus giving arepresentation of the digital values for particular points on thewideband video return 60, the sampling point being indicated as dots onthe wideband video 60. To develop the range error voltage V the data isprocessed by summing the first n/2 samples, (1) and the last n/2samples, 4: from R, the stored data array. The difference of these twosums will be the range error voltage, V which represents the error incentering the n samples of R about the center of the target.

Part of the function of digital range tracker will be to generate an AGCvoltage, V proportional to the average amplitude of the echo signal.This is used as a conventional automatic gain control in the IFamplifiers 22 to keep a constant average angle sensitivity for stableclosed loop tracking. To perform the generation of the V a digital rangeerror detector processor 44 and 46 in FIG. 1 performs an additionalstep. As described above it subtracts the output from its summation ofcells 1 through n/2 and its summation of the cells n/2 plus 1 to n toobtain range error. To obtain V the number proportional to averageamplitude it adds these two summations.

Although the AGC process keeps the angle sensitivity constant for theoverall target the sensitivity within each resolution cell can fluctuateso that the angle video voltage in each cell is equal to the product ofA, the amplitude of the echo within the i" resolution cell and 0, in theactual angular displacement of the part of the target within the cellfrom the antenna axis. The resultant processed high range resolutionangle error voltage is equal to the summation n 2 Am: i=1

when there is no AGC. However, a normal AGC function effectively dividesthe error voltage by II 2 Ai i=1 to maintain constant average anglesensitivity giving a resultant angle error voltage V A This bydefinition is the true center of the target. In FIG. 3 it is seen thatthe data array R 62 is divided into two equal segments, one segment issummed from i l to n/2 while the second segment n/2 l is summed to n.These individual segments are subtracted from each other to obtain rangeerror video while when added together they produce an AGC voltage. Againreferring to FIG. 3 the early segment of data is stored and thenpresented to a range bin selector section 64A and the late section issupplied to range bin selector section 64B. These outputs are summed inearly adder 66 and the late adder 68, whose outputs are supplied one toa range voltage error output subtractor 70 and also to an adder 72 whichsupplies the AGC voltage.

Referring now to the digital error processing technique of one angle, inthis case the azimuth, angle wideband azimuth video return is shown inFIG. 4 as bipolar wide range video 80. This azimuth video is sampled andstored. A graphic representation 82 shows the bipolar sample and storagepoints at various time intervals, dots on line, of the amplitude of thewideband at video return 80. This stored information is supplied to anadder 84 which adds the sum of the A, element from 1 through n therebyproducing a digital average of the angular data obtained a positive ornegative voltage being indicative of the angle error of the antenna axisin azimuth. Of course the elevation portion of the processor will be thesame as the azimuth portion.

One purpose of using high range resolution monopulse tracking radar isto reduce the target angle scintillation. The scintillation is caused byintermodulation between the parts of the targets. The high rangeresolution technique resolves targets into many parts and measures theprecise angle of each part such that by means of being resolved theycannot intermodulate. The angle of each resolved part of the targetrelative to the antenna axis is indicated by a bipolar video voltage.The video amplitude is proportional to the angular displacement of thepart of the target from the antenna axis, the polarity indicates thedirection such azimuth right or left error indicated by a positive or anegative voltage respectively. Of course this digital weighted averagewhich is the output of adder 84 is supplied to a digital to analogconverter which supplies an analog output to control the antenna. Thiswould be the case in both the elevation and azimuth servo loops. Theprocessing of the elevation error signal V would be defined as follows:

where E, is the i element of an array E. The array, E, is arrayrepresenting a collection of samples taken from the wideband elevationvideo return just as array, A, represents the azimuth video, as shownabove.

Referring now to FIG. 2, a more detailed diagram of the digitalprocessor 32 of FIG. 1, it is seen that the wideband range video issupplied to an 8 channel high speed sample and hold circuit 90. Thesampling circuit 90 is triggered by a trigger shown as T which wouldnormally be supplied by the system timing circuits 36 of FIG. 1. Likethe sample and hold circuit the azimuth video is supplied to sample andhold circuit 92 and the elevation video is supplied to sample and holdcircuit 94. The outputs of all three of these are in turn supplied to amultiplexer 96. This multiplexer 96 in time supplies each of the sampledsignals to an A to D converter 98 which converts the analog signalslevel to digital signals which are stored in the general digitalcomputer 100 a portion of the digital processor. Converter 98 suppliesthe signal to the general purpose computer 100 which is comprised of amemory 102 and general purpose processor 104. These components combineto operate on signals in the manner described above. The V range errorsignal output is supplied to a D to A converter 106. The V output issupplied to a D to A converter 108 and the analog outputs for the V andV error signals are supplied by D to A converters 110 and 112,respectively. As shown in FIG. 2, should the equipment be able to usethe digital outputs this could be used directly by a particular portionof the radar system for control.

Now referring to FIG. 5, which is a comparison of the digital techniqueof the invention with the conventional split gate or early-late gaterange tracking system. Looking to the split gate range tracking, forpurposes of discussion a single video pulse is shown as being equallydistributed on either side of a center line. Thus we have equal areas,area A and area B. The normal range gate system would have a main rangegate approximately centered on the video return. This is shown as gate122. This gate causes the generation of an early gate for the first halfand a late gate for the second half of the main gate. These early andlate gates allow a portion of the range video 120 to charge on the earlygate to a certain potential and then is discharged to a second potentialby the late gate. The difference between these potentials is the errorrange output which is used to correct the trigger initiating range gate122. Now referring to the digital portion, an area is divided into twoareas A and B of equal amount and a wideband video of a monopulse radaris shown. This is approximately centered in a sampling window 32. Duringthe first half of the sampling window, the first portion of data array Ris sampled and stored. The second half of the window samples and storesthe remainder of the data array R. This data is digitally summedseparately and then the difference between the two sums is calculated.Therefore a voltage proportional to the difference between area A andarea B may be generated which is the range error signal. This signal maybe used to adjust the trigger initiating the sampling window 132 suchthat it is centered at the true center of the wideband video whenreceived.

Thus it is seen from what is described here is a new radar trackingsystem which employs digital processing to obtain range error signal,automatic gain control signal, azimuth error signal and elevation errorsignal. Thus the radar is able to reduce target angle scintillation. Themajor advantage of the invention described is to allow a tracking radarto track the true center of a target greatly reducing targetscintillation. This is possible since a target is composed of multiplereflectors and this invention allows the radar to track the centroid ofthe many reflectors weighed by the amplitude of each instead of trackingthe target as if it were a single reflected element.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore understoodthat within the scope of the appended claims, the invention may bepracticed otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

l. A radar tracking system which employs digital processing techniquesin target tracking:

antenna means functioning to provide, first, second and third signals;

said first signal being indicative of target range;

said second signal being indicative of target azimuth angular error fromthe boresight of said antenna; said third signal being indicative oftarget elevation angular error from the boresight of said antenna; meansfor digitizing the analog information of said first, second and thirdsignals and for forming first, second and third digital arraysrepresentative of said first, second and third signals, respectively;all said digital arrays having a length greater than 2;

a first digital array which is representative of said first signalhaving a length n which is an even number;

means for dividing said first digital array onto two equal halves;

means for individually summing said two equal halves separately;

means for deriving a first error signal which is representative of thedifference between the sums of said two equal halves;

thereby forming a range error signal.

2. The radar tracking system of claim 1 wherein said second digitalarray, which is representative of said second signal, is summed therebyforming an azimuth error signal to control said antenna.

3. The radar tracking system of claim 1 wherein said third digitalarray, which is representative of said third signal, is summed therebyforming an elevation error signal to control said antenna.

4. The radar tracking system of claim 1 wherein the two equal halves ofsaid first digital array are summed thereby forming an AGC signalproportional to the average amplitude of said first signal.

1. A radar tracking system which employs digital processing techniquesin target tracking: antenna means functioning to provide, first, secondand third signals; said first signal being indicative of target range;said second signal being indicative of target azimuth angular error fromthe boresight of said antenna; said third signal being indicative oftarget elevation angular error from the boresight of said antenna; meansfor digitizing the analog information of said first, second and thirdsignals and for forming first, second and third digital arraysrepresentative of said first, second and third signals, respectively;all said digital arrays having a length greater than 2; a first digitalarray which is representative of said first signal having a length nwhich is an even number; means for dividing said first digital arrayonto two equal halves; means for individually summing said two equalhalves separately; means for deriving a first error signal which isrepresentative of the difference between the sums of said two equalhalves; thereby forming a range error signal.
 2. The radar trackingsystem of claim 1 wherein said second digital array, which isrepresentative of said second signal, is summed thereby forming anazimuth error signal to control said antenna.
 3. The radar trackingsystem of claim 1 wherein said third digital array, which isrepresentative of said third signal, is summed thereby forming anelevation error signal to control said antenna.
 4. The radar trackingsystem of claim 1 wherein the two equal halves of said first digitalarray are summed thereby forming an AGC signal proportional to theaverage amplitude of said first signal.